2014 E250 CDI Limp Mode Issues without Any Faults

[JoeHorner]

I read on here lots of talk of "limp mode" but no fault code. It is quite possible you have some failure that is causing a torque limitation without a fault, I would not call that "limp mode".

Agree with rf065, it's often more than a simple torque limit.

On mine with a bad Baro sensor, it decided the MAP reading couldn't be trusted (as you suggest) and set an appropriate (it thought) fault code for MAP plausibility which doesn't show on standard scanners. But the response was to:

* disable the turbo entirely
* change into the highest gear possible as soon as possible (IE: 4th by about 25mph, 5th by 30 or so!)
* disable kickdown entirely
* limit revs to 2000rpm

That's considerably more than a simple torque limit and driving something this weight in a high gear with, effectively, a normally aspirated low-speed diesel c.1960s style fits a description of "limp" pretty well!

Also agree completely that xentry is the way to go. Hugely more versatile than any aftermarket kit, even "brand specific" stuff.

 

[supernoodle]

ECU software runs in the torque domain, ie setpoints are based on torque. You press the pedal and that linear position is translated into a driver torque demand. That torque demand and speed are then used to derive setpoints for boost pressure, EGR, rail pressure, injection pattern and timing. Along the way those demands can all be modified based on sensor feedback, eg temperatures, pressures. Those same sensor feedbacks can be used to create limitations to this demanded torque, eg the smoke limitation based on MAF I used earlier. The limitation could also come from a fault limit map, which can be applied when certain faults are active.
As a last step in the path that limited driver demanded torque is converted into a fuel quantities, which themselves are just opening times of the injector..

Torque limitations are not just single values but maps, and there will be maps that for given speed can limit the torque. So if that limit the torque at 3000rpm to 0Nm, you won't go above 3000rpm. Hence my use of term torque limitation.

Joe, your case was different to those who complain of this "limp mode" with no fault as you had a fault.
When there is a fault you can specify a series of resulting reactions. In your case ECU couldn't trust MAP so not suprising it wouldn't allow closed loop boost control, probably disabled open loop boost control too, applied an error torque map, etc. So in your case limp mode is fitting as ECU had intentionally reacted to a fault.

There can be 0 reactions to a fault, there may be 50. Whatever is deemed necessary for the engine to still run safely.

 

[JoeHorner]

Agree with all of that, although the fault code set in my case was only shown by a car specific (icarsoft) scanner as a minimum and xentry by preference. Generic OBD scanners didn't show anything, which could well be what many others are seeing.

Regardless, the technical difference between what happens and a "real" limp mode is a little academic when you're crawling up a fairly mild hill like an asthmatic Bedford 4 tonner and the guy in a 1.0 Micra behind you is getting annoyed (yep, been there....)